Generally avoid moving next to an unoccupied village. An enemy unit may move onto the village and attack you, while enjoying the defense and healing of the village.

You can use units from a previous scenario by selecting 'Recall' in the game menu. By recalling the same units over and over, you can build up a powerful and experienced army.

In most campaigns you will receive a gold bonus for finishing early, depending on the number of villages on the map, and the number of turns you finish early. You will always earn more gold this way than through capturing villages and waiting for turns to run out.

Units are healed when they advance a level. Used wisely, this can turn a fight.

Use healers to support your attacks - they will win you battles without needing to attack anything themselves.

Konrad: Can Merle retaliate from my attack? when attacking Merle, an Elvish Shaman

Delfador: Whenever a unit is attacked, it retaliates with one of their own weapons. This means that after each blow from the attacker, the defender retaliates with one blow. This continues until one side runs out of blows, in which case the other unit continues attacking until its blows are depleted as well. However, any unit can only retaliate by using a weapon of the same range as the one used to attack. This means that a ranged attack can only be countered by a ranged attack and a melee attack can only be countered by a melee attack. Most units have more powerful melee attacks then ranged attacks. Units with strong ranged attacks are useful in retaliation against melee oriented units. Some melee units without ranged attacks won't be able to retaliate at all. However ranged units cannot shoot across multiple hexes; this is one of the distinctive features of Wesnoth.

I've never pushed so far, we are crossing the border of the Black Eye lands. -- the first unit to move five hexes left of the keep

You are right, my son. We will help our orcish friends. But take care of you ... I would be desperate if something would happen to you. -- Blemaker to his son Grüu, both trolls, after they decide to help the orcs fight the dwarves (If Grüu dies, then you lose the scenario.)